Preparing for the final week of their off-season program and mandatory minicamp, the Ravens appeared all but set on the offensive line, at least in terms of who would be competing for the starting left guard position.

The battle between rookie Kelechi Osemele and second-year lineman Jah Reid — both converting from the tackle position — to replace former Pro Bowl guard Ben Grubbs was shaping up to be the most compelling competition of the preseason. However, that all changed on Friday when Baltimore announced the signing of former Bengals guard Bobbie Williams to a two-year contract.

Many assume it to be a move reflecting a lack of faith in Osemele and Reid as both have faced physical challenges during organized team activities, but coach John Harbaugh insisted the addition of Williams was something the Ravens were considering since the start of free agency. While unsuccessfully courting Philadelphia guard Evan Mathis to replace Grubbs, general manager Ozzie Newsome and the front office quietly monitored the rehabilitation of Williams’ fractured ankle suffered in Week 14 last season.

“He is a guy that we knew about right away,” Harbaugh said. “We have always liked him. We had him highly ranked. He had a situation with a broken leg last year. We had to see how that developed.”

With Osemele sidelined and left tackle Bryant McKinnie being held out of minicamp workouts due to conditioning concerns, Williams has immediately stepped in at left guard while Reid has played right tackle as Michael Oher has moved back to the left side of the starting offensive line this week. When the Ravens have their full allotment of linemen at the start of training camp, it remains to be seen how serious they are about entrusting a 35-year-old coming off a major injury to handle the left guard spot.

Despite having started 130 games in his 12 NFL seasons and reportedly receiving an $800,000 signing bonus, Williams hasn’t been assured of anything other than an opportunity to compete with the young linemen to nail down the starting left guard job. He has plenty to prove after playing only nine games last season after facing a four-game suspension for violating the league’s policy on performance enhancing substances and missing the final three games due to injury.

“That’s just it, that [the left guard spot is] open,” Williams said. “They like the guy I am. They like my character and they like my play. They said that I would be a great fit, and I believe that. I’m up for the challenge, and I’m going to try to maximize every chance I get.”

With the additions of Williams and former Rams and 49ers interior lineman Tony Wragge in the last week, offensive line coach Andy Moeller and Harbaugh have more options in formulating their starting five for the regular season.

It may look much different than anyone expected it to even a few weeks ago.

With the decision to keep McKinnie off the practice field, it’s clear the Ravens aren’t pleased with his weight and conditioning less than six weeks from the start of training camp. If they determine they cannot count on him to start on the left side, they could elect to move Oher permanently back to the left side and shift the competition between Osemele and Reid to right tackle — with Williams then starting at left guard.

A more likely scenario is a competition between Williams and Osemele at left guard with Reid shifting back to his natural tackle position as a reserve. Even if McKinnie sheds more weight and brings his conditioning to a satisfactory level, the Ravens are thin at tackle behind McKinnie and Oher with 2010 sixth-round choice Ramon Harewood struggling mightily during OTA workouts.

Not only does the addition of Williams give the Ravens another veteran on which to lean, but it creates many more combinations for a unit trying to fill the void left behind by its best offensive lineman in recent years.

“We are just building an offensive line. We’ll have competition,” Harbaugh said. “Those young guys are going to be in there competing. The best guys are going to play. It’s nice to have another veteran guy in the mix. It makes us stronger. We’ll see how it shakes out as we go.”

Williams will need to prove he is fully recovered from the ankle injury that diminished his value on the open market earlier this offseason as well as make the transition from right guard to the left side. It was a move interior lineman Andre Gurode struggled with last season trying to fill in for the injured Grubbs after he had spent his entire career at center and right guard.

When posed the question of how easily he could make the move, Williams expressed little concern as he prepares for his 13th season out of the University of Arkansas. The 6-foot-4, 345-pound guard is prepared for anything at this stage of his career, especially if it means a better opportunity to win a Super Bowl as he stays in the AFC North.

“At this stage in the game, that’s all it is is a change — just switch up your footwork and go with it,” Williams said. “The mentality is still the same.”

And it’s a mentality the organization would surely like to see the longtime Bengal bring with him to Baltimore.